1. Field of the Invention
The present invention relates to photographic elements bearing pressure-sensitive adhesive layers protected by overlaying peelable film layers. The invention in particular relates to the preparation of such elements by melt extrusion of the film onto the adhesive layer.
2. The Prior Art
Self-adhesive or pressure-sensitive adhesive photographic elements have gained widespread acceptance in the present day. Their use extends to the phototypesetting industry in which developed photographs are normally mounted adhesive side down onto so-called composition or mounting boards. The mounted photographs, moreover, can be removed and re-positioned until a satisfactory composition of photographs along with other material is achieved. Similarly, in the area of amateur photography, photographs can be mounted in albums by means of pressure-sensitive adhesives as in the phototypesetting industry.
Both of the above areas of use, as well as others, at some point in time require the application of adhesive to the photographic element, a potentially messy and inconvenient procedure. As a convenience to the user, manufacturers of photographic elements have precoated, for example, pressure-sensitive adhesives onto the paper support for photographic elements. To preserve the adhesive qualities of the adhesive layer, avoid premature sticking of the coated photographic elements to each other or unwanted surfaces, and prevent contact of the adhesive layer with photographic processing baths encountered by the element before use, protective layers, typically polymeric films, are applied over and in contact with the adhesive layer. The protective layer can then be removed at a chosen time to expose the pressure-sensitive adhesive layer for mounting as desired. See British Pat. No. 1,101,608 to Sandoz Ltd., published Jan. 31, 1968 for a description of such protective layers on pressure-sensitive layers of photographic elements. The subject of protective layers for pressure-sensitive adhesive layers on photographic elements is further disclosed in Dickard U.S. Pat. No. 3,135,608 issued June 2, 1964. In the latter patent, conventional release paper is laminated to the pressure-sensitive adhesive layer of a photographic element as discussed above. Particular protective layers with which the invention described below is concerned comprise ethylene polymers, such as polyethylene. The use of such materials has long been established in the photographic industry.
Techniques that are known to exist for applying protective layers onto pressure-sensitive adhesive layers, include lamination, solvent coating, in-situ polymerization, and extrusion. Of these, lamination or extrusion are often employed in the application of ethylene polymers. In contrast to extrusion coating of a polymer layer directly onto a substrate, the process of lamination is necessarily preceded by a preformation step which includes off-line extrusion of a film and distinct storage thereof in roll form on a tubular core. Lamination subsequently includes unwinding the preformed film and bringing it into cold contact with the adhesive coated substrate -- in this instance, photographic paper base. At first blush, the procedure of lamination may not seem undesirable. Off-line formation of the film, however, is inherently more costly in terms of film storage, winding operations, and the need for separate operating space, when compared to direct extrusion onto the substrate. Equally significant, the thickness of film that must be used in lamination is quite large by comparison to the thickness of film applied by extrusion. Lamination of thin film, such as 1 mil thick film, is quite difficult, often resulting in a wrinkled surface. Wrinkling, in turn, creates additional problems when the substrate with film protected adhesive layer is stored, emulsion coated, and processed in photographic baths. Such is not the case with extrusion, as the molten extruded film has a much greater tendency to maintain surface uniformity upon contacting the adhesive surface of the substrate.
The direct extrusion of ethylene polymers onto pressure-sensitive adhesive coated substrates has encountered several problems related to the temperature of extrusion, the ease with which the polymer layer can be detached from the adhesive layer when mounting is desired, and the line-speed capacity of extrusion.
It will be appreciated, that if the bond between the ethylene polymer layer and the adhesive layer is excessive, any attempt to detach the film layer as desired will result in the rupture of either the polymer film layer or the paper substrate on which the adhesive is coated. Empirically it has been found that the stripping force to overcome that bond should not exceed about 400 grams as determined by the stripback test described in greater detail hereinafter.
Furthermore, as the temperature of extrusion is lowered, the coherent strength of the ethylene polymer "curtain" before striking an advancing substrate is significantly reduced. As a consequence, the polymer curtain has a greater propensity to tear unless the speed of the advancing substrate is significantly lowered, in turn adversely affecting the economic success of the operation. The temperature at which the strength of the curtain becomes unacceptable depends in part on the melt index, or viscosity, of the material extruded, the more viscous materials requiring higher extrusion temperatures to avoid curtain tearing. As a general matter, however, ethylene polymers having melt indices, as later defined, within the range from about 3 to 14 cannot be extruded at 450.degree. F (232.degree. C) or lower without sacrificing line speed.
On the other hand, extrusion of the ethylene polymer layer at high extrusion temperatures such as above 580.degree. F (304.degree. C), while providing adequate coherent strength to the curtain and permitting higher line speeds, can result in bond strengths between the ethylene polymer layer and adhesive layer in excess of 400 grams. Furthermore, in extrapolating the results of high temperature extrusion to determine what might happen on decreasing the extrusion temperature, data indicated that a bond strength for permitting strippability of the ethylene polymer layer (i.e., a bond requiring less than a 400 gram force as described herein) would not have been expected at extrusion temperatures above about 450.degree. F. Hence it appeared that extrusion of ethylene polymers onto pressure-sensitive adhesive layers to produce 400 gram or less bond strengths would have to be carried out at 450.degree. F or lower extrusion temperatures at unacceptably low line speeds.